Potential of Zero Charge-Based Capacitive Deionization

ABSTRACT

The invention is a capacitive, aka electrostatic, deionization apparatus and method that solves the problem of short lifetime of conventional capacitive deionization (CDI) and of membrane capacitive deionization (MCDI) devices and methods by shifting the Potential of Zero Charge of electrode surfaces through surface modifications. Such electrode surface modifications provide very long lifetime capacitive deionization devices and methods.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/757,209, filed 3 Dec. 2015, which is a non-provisional application ofU.S. provisional patent application No. 62/195,578, filed 22 Jul. 2015,and 62/086,857, filed 3 Dec. 2014; all of the foregoing applications areincorporated herein in their entirety by reference.

FEDERAL FUNDING AND JOINT RESEARCH AGREEMENT

The research leading to the invention disclosed herein was partiallyfunded by U.S. Dept. of Energy contract no. DE-PI0000017, which contractprovides partial funding for a Joint Research Agreement among theUniversity of Kentucky, West Virginia University Research Corporation,Huazhong University of Science and Technology, and 26 additional partieslisted at http://www.us-china-cerc.org/Advanced_Coal_Technology.html.

BACKGROUND OF THE INVENTION Field of the Invention

The field of the invention is capacitive, aka electrostatic,deionization devices and methods used to remove salt and other ions fromsolutions.

Definitions

“Adsorption” means attracting ions in an input stream to, and retainingthose ions on an electrode surface.

“aMCDI” means a CDI cell in which each electrode is surrounded by amembrane and in which either or both the anode and cathode containsurface-charge enhanced surfaces.

“AMX-CDI” means a CDI cell in which each anode is covered by an anionexchange membrane while the cathode remains uncovered.

“AMX-aCDI” means a CDI cell in which each anode is covered by an anionexchange membrane while the cathode is uncovered and contains negativesurface charge.

“BET surface area” means surface area determined by theBrunauer-Emmett-Teller method, which is a physical adsorption-basedmethod using nitrogen to determine the surface area of a material.

“Capacitive deionization” means removing ions from an input stream to acell by adsorption, and passing the deionized stream to the cell output.

“Capacitive deionization cell” means a cell that uses electrostaticforces to adsorb ions from an input stream. In a “traditional” or“conventional” capacitive deionization cell, a positive voltage isapplied to an anode electrode, and a negative voltage is applied to acathode electrode, to cause adsorption of negative ions to the anode andpositive ions to the cathode while the voltages are applied.

“Cell” means a plurality of electrodes exposed to an input stream, withan outlet for the output stream/waste stream, a short-circuit switch orpower supply attached to the electrodes, and a means of controlling thepower supply. A cell can optionally include a means of controlling theinput stream and the output stream/waste stream.

“Charging potential” means a voltage applied to, or inherent in surfacefunctional groups of, an electrode of a cell and which causes movementof ions in the input stream of a cell to an electrode.

“CMX-CDI” means a CDI cell in which each cathode is covered by a cationexchange membrane while the anode remains uncovered.

“CMX-aCDI” means a CDI cell in which each cathode is covered by a cationexchange membrane while the anode is uncovered and contains positivesurface charge.

“Conductivity” means the electrical conductivity of an input stream,output stream, or a waste stream. Conductivity is a surrogatemeasurement for the molarity of ions in an output stream or wastestream. Conductivity is directly proportional to molarity of ions insuch streams.

“Co-ion” means, in a CDI cell, an anion that is attracted to a cathodewhen the cathode's potential is higher than its E_(PZC) and a cationthat is attracted to an anode when the anode's potential is lower thanits E_(PZC).

“Counter-ion” means a negative ion that is attracted to a positivelycharged electrode and a positive ion that is attracted to a negativelycharged cathode.

“CX” means carbon xerogel. CX electrodes possess a mesoporous structurewith a nominal surface area of ˜200 m²/g.

“Cycle” means a cycle of operation, adsorption followed by desorption,of a capacitive deionization cell.

“Deionization” means removing ions in an input stream by adsorption toan electrode surface and passing the deionized stream as output.

“Deionization cell” means a cell that removes ions from an input stream.Deionization cells are of various types, e.g., traditional, MCDI, aMCDI,i-CDI.

“Desorption” means releasing adsorbed ions from an electrode and into awaste stream.

“Discharging potential” means a reduced or reversed polarity voltageapplied to, or inherent in surface functional groups of, an electrode ofa cell to cause desorption of ions from the electrode into a wastestream.

“Electrode” means a material, typically porous carbon, which iselectrically conductive.

“i-CDI cell” means an “inverted” capacitive deionization cell accordingto the invention disclosed herein.

“E_(PZC)” or “potential of zero charge”, mean the potential of anelectrode at which there is a minimum in ion adsorption at the surface.

“E₀” is the potential vs. a reference electrode of a capacitivedeionization cell when the electrodes are short-circuited (i.e., E_(o)is the potential during a short-circuit condition).

“Flow rate” means the flow rate, typically in L/hr, ml/min, etc., of aninput, output, or waste stream.

“Input stream” means a liquid, typically water containing various ions,admitted to a cell.

“MCDI cell” means a CDI cell in which each electrode is surrounded by amembrane.

“Membrane” means a carbon or carbon-based fabric or coating affixed orapplied to an electrode.

“N-” means negative surface charge, e.g., N-CX means a carbon xerogelelectrode with net negatively charged surface groups.

“Output stream” means a liquid that has passed through an adsorbingdeionization cell and contains a lower molarity of ions than in theinput stream.

“P-” means positive surface charge, e.g., P-CX means a carbon xerogelelectrode with net positively charged surface groups.

“pH_(PZC)” means the pH of a solution at a given E_(PZC) determined byvarying the pH of the solution.

“Polarization window” means the span or range of potentials/voltagesused to conduct deionization (adsorption) and regeneration (desorption)of a capacitive deionization cell.

“Polarity” means the polarity of a DC voltage, either positive ornegative.

“Pristine” in reference to electrodes means without surfacemodifications; for example, a Spectracarb electrode, as supplied by themanufacturer, is pristine.

“Purify” means to remove ions from an input stream. Purificationincludes water softening, i.e., the removal of calcium, magnesium, andcertain other metal cations in hard water.

“Relocation” of an E_(PZC) is a change in potential (aka “location”) ofthe E_(PZC), as shown in a cyclic voltammogram, of an electrode byaccumulation of adsorption/desorption cycles.

To “shift” or “position” of an E_(PZC) means to alter the potential (aka“location”) of the E_(PZC) of an electrode by intentional chemical orelectrochemical modification of the electrode surface.

“SC” means Spectracarb carbon electrode, a carbon electrode commonlyused as a reference electrode, e.g., in cyclic voltammetry.

“SCE” means a saturated calomel electrode, a standard referenceelectrode.

“Si-CX” means silica-coated carbon xerogel.

“Surface-charge enhanced surface” means an electrode surface that hasbeen treated.

“Treat” means to modify an electrode surface to shift the E_(PZC) of theelectrode as disclosed herein.

“Untreated” means an electrode without an electrode surface modificationdisclosed herein, i.e., a pristine carbon electrode.

“Voltage” and “potential” are synonymous herein. Voltage is directcurrent (“DC”) unless otherwise specified.

“Waste stream” means a liquid that has passed through a desorbingdeionization cell and contains a higher molarity of ions than in theinput stream.

“Zeta potential” is the potential difference between a dispersion mediumand the fluid surrounding the dispersed particle.

Related Art

As population increases and water demand continues to rise around theglobe, access to potable drinking water will also increase inimportance. To meet water standards for consumption, agriculture, powerplants, or one of many other uses, numerous water purity conditions mustbe met, including level of salt content. Salt content is difficult toseparate by conventional filtration methods due to the small molecularsize of most salts. Many other dissolved ionic compounds are alsochallenging to separate via typical chemical precipitation routes.Dissolved ions such as Na⁺, K⁺, Ca²⁺, Fe²⁺, Fe³⁺, Cu²⁺, Mg²⁺, Cl⁻, SO₄²⁻, and NO₃ ⁻ are commonly found in water sources and require specificseparation methods in order to produce purified water. Conventionalseparation methods include multi-stage flash distillation (MSF) andreverse osmosis (RO). In MSF, water is separated from a salt waterstream using a distillation process, wherein water is boiled and vaporcollected to produce a pure water stream. While effective, this methodis quite energy intensive and suffers from equipment corrosion issues.In RO, very small pore sizes are used to separate slightly smaller watermolecules from larger hydrated salt molecules. This process requires apressure gradient to overcome the osmotic pressure of the salt solutionand to transport water across the semipermeable membrane to a purifiedpermeate stream. While RO is typically more efficient than MSF, itrequires pumps capable of higher pressures and is subject to organic,biological, and precipitation-based fouling of the membrane surface,which ultimately limits the lifetime of the separation process andincreases the expense, especially at municipal scale.

Capacitive deionization (CDI) is an emerging separation process thatrelies on the use of electrostatics to separate dissolved salts fromwater/aqueous solution. In conventional CDI, an electrical potential isapplied to a pair of (typically) carbon electrodes with the anodedefined as the electrode to which a positive potential is applied andthe cathode as the electrode to which a negative potential is applied.In this conventional CDI process, negative ions or anions, such as Cl⁻and SO₄ ²⁻, are attracted to the positively-polarized electrode (anode)while positive ions or cations, such as Na⁺ and Ca²⁺, are attracted tothe negatively-polarized electrode (cathode) as shown in FIG. 1. Whenthe carbon electrodes are saturated with salt/ions, the appliedpotential is reduced, short-circuited, removed, or reversed (eithermanually or under computer control) to desorb these ions into aconcentrated waste stream. When the cell potential is reduced,short-circuited, or removed, the driving force for ion adsorption issubsequently reduced at the carbon surface, resulting in ion desorption.Desorption (removal of ions from the electrode surface) regenerates thecarbon for adsorption/separation of more ions after voltage is reappliedto the electrodes (desorption of the electrode surface is not completein a traditional CDI, as explained below). To accelerate desorption, thevoltage polarity applied to the electrodes can be reversed: a reversedpolarity (negative to anode, positive to cathode) voltage on theelectrode surface will repel co-ions, resulting in faster ion desorptioncompared with an open circuit, short circuit, or reduced voltage ofnon-reversed polarity. Initial applied potentials for adsorption aretypically between 0-2.0 V, but values up to 3.0 V have been reported. Inthis manner, salt can be periodically removed from solution and desorbedinto a concentrated waste stream.

Separation of Aqueous Sodium Chloride with Traditional CDI

While this process sounds relatively simple, in actuality, theseparation process is more complex, e.g., changing surface electrodeproperties substantially changes the salt separation process. The carbonelectrodes used in a CDI process are typically designed to be inert,high surface area (i.e., very porous), and conductive electrodes for theadsorption and desorption of ions while being modulated by an appliedelectrical potential. The application of an electric potential to acarbon electrode changes the properties of most carbon electrodes. Asshown in FIG. 2, initially, salt is adsorbed in a CDI cell with anapplied electrical potential (shaded regions) with conductivity of theoutput stream decreasing with an applied potential (a decrease inconductivity of the output stream means that total adsorption of ions isincreasing) and then desorbed when the cell is short-circuited(non-shaded regions). There is a peak when a voltage is applied (aninitial spike to a lowest 6 (conductivity) upon application ofpotential, shown in the shaded areas of FIG. 2) and a reverse peak whena voltage is removed (or reversed or decreased, shown by a spike to ahighest 6 (conductivity) upon short circuit of the electrodes in FIG. 2,shown in the unshaded areas of FIG. 2); FIG. 2 shows the alternateapplication of a charging potential to the electrodes (shaded areas) andthen a short circuit of the electrodes (unshaded areas)). Thisexperiment was carried out using ˜4 g of carbon xerogel electrodes, 1.5mm silicone spacers, 2 L of 4.3 mM N₂ deaerated NaCl solution,adsorption and desorption times of 30 min each, a charging potential of1.2 V, a discharging potential of 0 V (short circuit), and a flow rateof 75 ml min⁻¹. FIG. 2 shows the first 7 cycles of a CDI process. Whileinitially stable, with repeated cycling, this adsorption-desorptionbehavior begins to change: the higher 6 (less reduction in conductivity)after a charging potential is applied means that far fewer ions arebeing adsorbed. Shown in FIG. 3 is the salt removal response of a CDIcell after 227 cycles. After 227 cycles, it is clearly evident that thesteady state concentration difference in conductivity, as reflected inan essentially flat conductivity, σ, profile, with and without anapplied potential: change in conductivity has decreased to almost zero,i.e., in both the shaded (1.2 V applied) and non-shaded (0 V, shortcircuited) regions, the steady state conductivity or level of saltcontent is nearly the same. The inversion peak upon applied voltage,denoted in FIG. 3, is indicative of inefficiency in the separationprocess that results in decreased net salt removal from the inputstream. This means that the CDI cell is no longer functioning as a saltseparation device: it has reached the end of device life and must bereplaced. When salt adsorption capacity (in mg of salt per gram ofcarbon electrode) is plotted as a function of cycle number, the resultis shown in FIG. 4. Salt adsorption capacity, Γ, is defined in equation1 as:

Γ=(ΔσMV)/(mc)  (1)

where σ is the conductivity difference with and without an appliedpotential, M is the molecular weight, V is the solution volume, m is themass of carbon electrodes, and c is a calibration constant for saltconcentration vs. conductivity. Clearly in FIG. 4, the salt adsorptioncapacity of the carbon electrodes to adsorb salt continually degradesuntil no separation is seen. This degradation process is not unique tothe carbon used in this example, but has also been shown for carbonxerogel, Spectracarb activated carbon cloth, and Zorflex® activatedcarbon cloth, making this degradation process almost universal forcarbon in a conventional CDI system. The charge efficiency, Λ, is ameasure of the separation efficiency of the overall process where onemole of electrons would remove one mole of salt in a perfectly efficientsystem. The charge efficiency is defined in equation 2 as:

Λ=(ΓF/M)/(Q _(ad))  (2)

where Γ is the salt adsorption capacity, F is Faraday's constant, M isthe molecular weight, and Q_(ad) is the total charge passed duringcharging. Shown in FIG. 5 is a loss in the charge efficiency as the saltadsorption capacity decreases to zero. In order to commercialize anelectrostatic separation (aka capacitive deionization) process, dramaticincreases in the cell lifetime are required. In an automotive analogy, aCDI cell gets terrible gas mileage and is not economically acceptable.If a CDI cell were implemented with a current CDI cell lifetime of 10days, and if each CDI cell costs $5000, the 2-year cell replacement costwould be $365,000, reflecting cell replacement every 10 days (butexcluding labor and administrative costs), an obviously unacceptablevalue. CDI cell life depends upon total cycles, so actual CDI cell lifecan be much shorter than 10 days.

Research in carbon electrodes has focused on the improving the porosityof carbon materials with which to construct supercapacitor electrodes,but research has not been conducted that would be particularly relevantto ionic separation and water purification. Known advancements to theconventional CDI system include new cell designs, asymmetric electrodecoatings, and the application of ion-exchange membranes. Probably themost successful advancement in capacitive deionization technology knownbefore the invention disclosed herein has been the addition ofion-exchange membranes to form membrane capacitive deionization (MCDI).MCDI cells provide not only a more stable separation process but alsohigher salt adsorption capacities. However, the addition of membranes tothe conventional CDI cell increases overall cell cost dramatically,making commercial success at a large-scale much more difficult, and doesnot address the underlying cumulative (aggregate cycle dependent)degradation process (i.e., short cell life, aka deactivation) for aconventional CDI cell shown in FIG. 4.

SUMMARY OF THE INVENTION

The technical problem to be solved is to provide a capacitive, akaelectrostatic, deionization apparatus and method that solves the problemof short lifetime of conventional capacitive deionization (CDI) and ofmembrane capacitive deionization (MCDI) devices and methods. Thesolution disclosed herein (i) properly characterizes previouslymisunderstood cumulative relocation of electrode E_(PZC)s and (ii)electrode surface modifications that provide very long lifetimecapacitive deionization devices and methods. Unlike prior art devices,the electrode surfaces of the “inverted capacitive deionization”(“i-CDI”) devices disclosed herein are restored after each desorption tominimum ion conditions (i.e., essentially as adsorptive as at the firstcycle of use), thereby providing capacitive deionization cells of vastlyimproved separation lifetime. Moreover, the discharging phase of thedisclosed method can generate electricity by discharge of capacitance ini-CDI cells. The results obtained from long-term CDI tests demonstratethe importance for obtaining targeted and stable surface chemistry inthe construction of an electrostatic-based separation cell.

The inventors herein disclose, for the first time, among otherembodiments, an electrostatic separation process (aka capacitivedeionization process) that utilizes pretreated, oxidized carbon anodesin the construction of a stable salt separation process, therebymitigating the degradation and short cell lifetime issues seen inprevious CDI and MCDI devices and methods. The invention disclosedherein utilizes improvements in “electrode surface charge” technology,specifically positioning (aka “shifting”) the “potential of zero charge”(E_(PZC)), in assessing and improving the capacity, charge efficiency,and cell life of capacitive deionization cells and capacitive-based ionseparation processes. The invention disclosed herein provides a cellwith salt separation efficiency for substantially longer time periodsthan that seen for prior CDI and MCDI cells. The inventors' research hasfocused on (i) electrode surface modification chemistry to improve thecharge storage capacity and charge efficiency of electrodes for ionicseparation and water purification, and (ii) how, and how much, to shiftthe E_(PZC) of anodes and cathodes. There has been no research, otherthan the inventors' publications, on modifying electrode surface chargeas a means of improving capacitive deionization technology.

A first embodiment of the invention comprises a structure comprising atleast one inlet, at least one outlet, at least one anode, at least onecathode, a switch operating to apply a short circuit or a userselectable DC constant voltage or constant current to at least one anodeand to at least one cathode, and with an ionic solution admitted throughthe inlet and discharged through the outlet, which ionic solution isdeionized by contact with at least one anode and at least one cathode,wherein the location of the E_(PZC) of at least one anode has beenshifted by modification of the anode surface to an increased E_(PZC).The modification of the anode results from a treatment selected from thegroup consisting of oxidation by exposure to acid, covalent attachmentof functional groups that are negatively charged when in contact withthe ionic solution and without voltage applied to the anode, covalentattachment of silica functional groups to an anode, attachment ofsulfonic acid groups, and attachment of any surface functional groupthat can result in negative zeta potentials in ionic solutions.

A second embodiment of the invention comprises a structure comprising atleast one inlet, at least one outlet, at least one anode, at least onecathode, a switch operating to apply a short circuit or a userselectable DC constant voltage or constant current to at least one anodeand to at least one cathode, and with an ionic solution admitted throughthe inlet and discharged through the outlet, which ionic solution isdeionized by contact with at least one anode and at least one cathode,wherein the location of the E_(PZC) of at least one cathode has beenshifted by modification of the cathode surface to a decreased E_(PZC).The modification of the cathode results from a treatment selected fromthe group consisting of reduction by exposure to reducing treatments byN₂, Ar, and H₂, covalent attachment of functional groups [that] arepositively charged when in contact with the solution and without voltageapplied to the cathode, covalent attachment of amine functional groupsto a cathode, reduced carbon surfaces including carbon basal planes,attachment of alumina surface species, and attachment of any surfacefunctional group that can result in positive zeta potentials in ionicsolutions.

A third embodiment of the invention comprises a structure comprising atleast one inlet, at least one outlet, at least one anode, at least onecathode, a switch operating to apply a short circuit or a userselectable DC constant voltage or constant current supply to at leastone anode and to at least one cathode, and with an ionic solutionadmitted through the inlet and discharged through the outlet, whichionic solution is deionized by contact with the at least one anode andat least one cathode, wherein the location of the E_(PZC) of at leastone anode has been shifted by modification of the anode surface to anincreased E_(PZC), and wherein the location of the E_(PZC) of at leastone cathode has been shifted by modification of the cathode surface to adecreased E_(PZC). The modification of the anode results from atreatment selected from the group consisting of oxidation by exposure toacid, covalent attachment of functional groups that are negativelycharged when in contact with the ionic solution and without voltageapplied to the anode, covalent attachment of silica functional groups toan anode, attachment of sulfonic acid groups to an anode, and oxidationtreatments from heating in an O₂ environment or electrochemically,wherein the modification of the cathode results from a treatmentselected from the group consisting of reduction by exposure to reducingtreatments by N₂, Ar, and H₂, covalent attachment of functional groupsare positively charged when in contact with the solution and withoutvoltage applied to the cathode, covalent attachment of amine functionalgroups to a cathode, reduced carbon surfaces including carbon basalplanes, and materials possessing positive zeta potentials in ionicsolutions.

In all embodiments, anodes, cathodes, and ionic solution are containedin a structure with an inlet through which the solution (input stream)is admitted and permitted to contact the anode(s) and cathode(s),penetrations of electrical conductors through the structure to permitoperation of a short circuit switch connected to the anode(s) andcathode(s) or application of user selectable voltage to the anode(s) andcathode(s), and an outlet in the structure through which deionizedsolution (output stream) or solution with desorbed ions (waste stream)is discharged from the structure. The structure is typically closedexcept for the inlet, outlet, and penetrations of electrical conductorsto effect short-circuiting of, or application of voltage to, theanode(s) and cathode(s). The structure and its contents are called adeionization cell. In alternate embodiments, the switch and an internalpower supply can be wirelessly controlled to avoid penetration of thestructure by electrical conductors.

The degree of purification (deionization) of an input stream iscontrolled primarily by the total electrode surface area in a cell towhich a given volume of input stream is exposed, the duration ofexposure for adsorption of ions before desorption, the voltage(potentials) or current modulated in the system, and the working voltagewindow (described below). Controlling the degree of purification(deionization) through modulation of the current supplied or drawn fromthe deionization cell, or through modulation of voltage supplied ordrawn from the deionization cell, enables fine adjustment ofdeionization.

Embodiments of the invention can be used to purify power plantwastewater, power plant cooling water, laundry wastewater, water to bepurified for human consumption, water to be purified for agriculture,water to be purified for horticulture, water to be purified for use infood, water to be softened, sea water to be purified for humanconsumption, water to be purified for laboratory use, brackish water tobe purified for human consumption or agriculture use, and water to bepurified for medical use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Capacitive deionization (CDI) process where salt isadsorbed/removed under the influence of an applied potential (suppliedhere by a power source) and desorbed when the potential is reduced,short-circuited, removed, or reversed. The cell on the right side ofFIG. 1 shows a short circuit of the anode and cathode, resulting isdesorption of ions from the electrodes.

FIG. 2. Adsorption (shaded) and desorption (unshaded) for a CDI cellcycled at 1.2/0 V for 7 cycles. Fluctuating signal indicated repeatedadsorption (shaded region, 1.2 V) and desorption (non-shaded region, 0V) events as the potential was applied to the electrodes and thenshort-circuited.

FIG. 3. Repeated cycling of the CDI cell shown in FIG. 2 after 227cycles at 1.2/0 V. Minimal conductivity differences are now seen betweenthe adsorption and desorption steps.

FIG. 4. Salt adsorption capacity, Γ, for a CDI cell cycled at 1.2/0 V.

FIG. 5. Charge efficiency, Λ, for a CDI cell cycled at 1.2/0 V.

FIG. 6. Cyclic voltammogram (CV) of a pristine carbon xerogel electrodein N₂ deaerated 4.3 mM NaCl solution at 1 mV/s.

FIG. 7. Cyclic voltammograms (CVs) of a pristine carbon xerogelelectrode as well as carbon xerogel electrodes used as positive (anode)and negative (cathode) electrodes in a CDI cell. These tests werecarried out in N₂ deaerated 4.3 mM NaCl solution at 1 mV/s.

FIG. 8. Potential distributions for the used electrodes according toFIG. 7. The E_(PZC) at the anode (E_(PZC+)) and the E_(PZC) at thecathode (E_(PZC−)) shown in the graphs have been averaged based upon theanodic and cathodic E_(PZC)s in the voltammogram. E₊ and E⁻ are thepotentials applied to the anode and cathode, respectively.

FIGS. 9A and 9B show FTIR (FIG. 9A) and surface acidity analysis (FIG.9B) of carbon xerogel (CX) samples with oxidation by nitric acid

FIGS. 10A and 10B show CVs of carbon xerogel (FIG. 10A) and of SCelectrodes (FIG. 10B) demonstrating positioning of the E_(PZC) at morepositive potentials through nitric acid treatments

FIGS. 11A and 11B show operation of an a conventional CDI cell (FIG.11A) compared to an inverted capacitive deionization (i-CDI) cell (FIG.11B) where adsorption of ions takes place at a short circuit potentialand desorption takes place with an applied potential.

FIG. 12. On the left, when the E_(PZC)s for both electrodes aredissimilarly located with respect to the short-circuit voltage (E_(o)),the potential window between the E_(PZC)s at the anode and cathode canbe used for desalination, but the adsorption-desorption behavior will beinverted. On the right, potentials distributed at the anode (E₊) andcathode (E⁻) at a total cell potential of 0.8 and 1 V in 4.3 mMdeaerated NaCl solution when a Si-CX anode and pristine CX cathode wereused in a four-electrode cell.

FIGS. 13A and 13B show conductivity (σ) (FIG. 13A) and current density(j) (FIG. 13B) for initial cycling (3^(rd)-5^(th)) for i-CDI and CDIsystems at 0.8/0 V in 31 L of 4.3 mM deaerated NaCl solution at 75 mLmin⁻¹.

FIGS. 14A and 14B show selected profiles of the conductivity (s) at the50-57^(th) cycles for i-CDI (FIG. 14A) and CDI systems (FIG. 14B) at0.8/0 V in 31 L of 4.3 mM deaerated NaCl solution at 75 mL min⁻¹.

FIGS. 15A, 15B, and 15C show salt adsorption capacity (Γ) (FIG. 15A),charge passed (Q) (FIG. 15B), and charge efficiency (Λ) (FIG. 15C) forthe discharging step for i-CDI and CDI systems discharging at 0 V. Inaddition, data for the CDI system used at 1.2/0 V was added into theplots for comparison.

FIGS. 16A, 16B, and 16C show salt adsorption capacity (Γ) (FIG. 16A),charge passed (Q) (FIG. 16B), and charge efficiency (Λ) (FIG. 16C) forthe discharging step for i-CDI and CDI systems charging at 0.8 V. Inaddition, data for the CDI system used at 1.2/0 V was added into theplots for comparison.

FIG. 17. Enhanced stability by the i-CDI system employed with a CXcathode and a Si-CX anode. This test was performed at 0.8/0 V in 31 L of4.3 mM deaerated NaCl solution. Comparisons in performance to standardCDI operation under similar conditions with pristine CX electrodes areshown. In this plot, regression lines have been added.

FIGS. 18A, 18C, and 18C show heat treatment in air/oxygen (FIG. 18A),acid treatments to oxidize the carbon surface (FIG. 18B), and a silicacoating method with TEOS (FIG. 18C), which all lead to carbon electrodeswith positively shifted E_(PZC)s (negative surface charges).

FIGS. 19A and 19B show tetraethyl orthosilicate (TEOS) treatment ofcarbon xerogel (CX) electrodes to yield silica groups on carbon xerogelelectrodes (CX/TEOS) to positively shift the E_(PZC) (FIG. 19A) andnitric acid treatment of commercially available Spectracarb (SC) toyield nitrate groups on carbon xerogel electrodes (SC/HNO₃) topositively shift the E_(PZC) (FIG. 19B).

FIG. 20. Amine functionalization of a carbon surface usingethylenediamine treatment of Spectracarb electrodes.

FIGS. 21A, 21B, and 21C show chemical characterizations of the pristineand treated SC. Fourier-transform infrared (FTIR) spectra of the samplesare displayed in FIG. 21A. These samples were further tested using 4.3mM NaCl solutions with different pH values to estimate point of zerocharge (pH_(PZC)) values, and shown in FIG. 21B. Cyclic voltammograms ofthe electrodes were carried out at 0.5 mV s⁻¹ in 4.3 mM deaerated NaClsolution and shown in FIG. 21C. The capacitance was calculated via thecurrent density divided by the voltage scan rate. The pH_(PZC) andpotential of zero charge (E_(PZC)) values are highlighted by arrows inFIGS. 21B and 21C.

FIGS. 22A and 22B show selected cycles when an i-CDI cell was configuredwith 16 sheets of P-SC cathodes and 16 sheets of N-SC anodes byconcentration (FIG. 22A) and by current density (FIG. 22B). These testswere performed at different [dis??]charging voltages for salt desorptionand a short-circuit voltage for salt adsorption (X/0 V, whereX=0.15-1.25) in ˜31 L of ˜4.3 mM deaerated NaCl solution at 20 mL min⁻¹.Each charging and discharging half-cycle took 4000 s.

FIGS. 23A and 23B show performance evaluations of the charging step forsalt desorption (FIG. 23A) and the short-circuit for salt adsorption fori-CDI cells configured with surface charge enhanced SC and CX electrodes(FIG. 23B). CX* and SC* designate i-CDI cells configured with pristinecathodes instead of P-CX and P-SC cathodes.

FIGS. 24A and 24B show a membrane capacitive deionization (MCDI) processin which salt is adsorbed/removed under the influence of an appliedpotential (supplied here by a power source) (FIG. 24A) and desorbed whenthe potential is reduced, short-circuited, removed, or reversed whereanion exchange membranes are placed over the anode electrodes and cationexchange membranes are placed over the cathode electrodes (FIG. 24B).

FIGS. 25A and 25B) show asymmetric membrane capacitive deionization(aMCDI) with positive surface charge enhanced anodes and negativesurface charge enhanced cathodes. FIG. 25A shows adsorption whilepotential is applied to the anode and cathode. FIG. 25B show desorptionwhen the anode and cathode are at a potential of 0 V.

FIGS. 26A and 26B show a comparison of CDI and MCDI performance.Conductivity profiles (FIG. 26A) and zoomed-in conductivity profiles(FIG. 26B). Cycle operation at 1.2/0 V with recirculation of 600 ml of 5mM NaCl solution in a batch mode setup formed with carbon xerogelelectrodes.

FIGS. 27A and 27B show a comparison of CDI and MCDI performance.Charging current (FIG. 27A) and electrosorption capacity over testingperiod (FIG. 27B). Cycle operation at 1.2/0 V with recirculation of 600ml of 5 mM NaCl solution in a batch mode setup formed with carbonxerogel electrodes.

FIGS. 28A and 28B show post-measurement of potential of zero charge(E_(PZC)) of used CDI carbon xerogel (CX) electrodes at the anode (FIG.28A) and at the cathode (FIG. 28B).

FIGS. 29A and 29B show post-measurement of potential of zero charge(E_(PZC)) of used MCDI carbon xerogel (CX) electrodes at the anode (FIG.29A) and at the cathode (FIG. 29B). MCDI suppresses shifting of E_(PZC).

FIGS. 30A and 30B show performance of CDI (FIG. 30A) and MCDI (FIG. 30B)cells configured with combinations of pristine and oxidized CXelectrodes. Cycle operation at 1.2/0 V with recirculation of 600 ml of 5mM NaCl solution in a batch mode setup formed with carbon xerogelelectrodes.

FIGS. 31A and 31B show electronic charge performance of MCDI cellsconfigured combinations of pristine and oxidized CX electrodes (FIG.31A) and zoomed-in current profile (FIG. 31B). Cycle operation at 1.2/0V with recirculation of 600 ml of 5 mM NaCl solution in a batch modesetup formed with carbon xerogel electrodes.

FIGS. 32A, 32B, and 32C show summaries of performance results for MCDIcells configured with combinations of pristine (Pr) and oxidized (Ox)electrodes used as anodes (legend “A”) and cathodes (legend “C”),including charge passed (FIG. 32A), salt adsorption capacity (FIG. 32B),and charge efficiency (FIG. 32C).

FIGS. 33A and 33B show pore distribution (FIG. 33A) and differentialcapacitance curves for E_(PZC) location (FIG. 33B) of pristine (Pr) andoxidized (Ox) Zorflex® (ZX) electrodes.

FIGS. 34A and 34B show conductivity (FIG. 34A) and current (FIG. 34B)profiles of MCDI cells formed with combinations of pristine (Pr) andoxidized (Ox) Zorflex® activated carbon as anodes (+) and cathodes (−).Cycle operation at 1.2/0 V with recirculation of 500 ml of 5 mM NaClsolution in a batch mode setup.

FIGS. 35A and 35B show long-term electrosorption (FIG. 35A) and chargeefficiency (FIG. 35B) performance of MCDI cells formed with combinationsof pristine (Pr) and oxidized (Ox) Zorflex® activated carbon as anodes(+) and cathodes (−). Cycle operation at 1.2/0 V with recirculation of500 ml of 5 mM NaCl solution in a batch mode setup.

FIGS. 36A and 36B show nitrogen adsorption of Spectracarb (SC)electrodes (FIG. 36A) and the E_(PZC) location of pristine and oxidizedSC electrodes (FIG. 36B).

FIGS. 37A, 37B, 37C, and 37D show performance of MCDI and aMCDI cellswith Spectracarb electrodes (SC) including conductivity (FIG. 37A), saltadsorption capacity (FIG. 37B), charge efficiency (37C), and chargepassed (FIG. 37D). Cycle operation at 1.2/0 V with recirculation of 1000ml of 5 mM NaCl solution in a batch mode setup.

FIGS. 38A, 38B, and 38C show conductivity (FIG. 38A), dissolved oxygen(FIG. 38B), and pH (FIG. 38C) profiles of CDI, MCDI, cationic membraneonly CDI (CMX-CDI), and anionic membrane only-CDI (AMX-CDI) cells formedwith pristine Spectracarb (SC) anode and cathode electrodes. Cycleoperation at 1.2/0 V with recirculation of 1000 ml of 5 mM NaCl solutionin a batch mode setup.

FIGS. 39A, 39B, 39C, and 39D show conductivity (FIG. 39A), dissolvedoxygen (FIG. 39B), pH (FIG. 39C), and current (FIG. 39D) profiles ofMCDI cell with pristine SC anode and cathode electrodes and anionicmembrane only-asymmetric CDI (AMX-aCDI) cells formed with pristine SCanode and oxidized SC cathode electrodes. Cycle operation at 1.2/0 Vwith recirculation of 1000 ml of 5 mM NaCl solution in a batch modesetup.

FIGS. 40A, 40B, and 40C show electrosorption capacity (FIG. 40A),electronic charge (FIG. 40B), and charge efficiency (FIG. 40C) ofconventional MCDI, aMCDI, CDI, and single membrane CDI cells. [twoAMX-aCDI traces in graphs]

FIG. 41. Projected capital and replacement costs for CDI, MCDI, i-CDI,and AMX or CMX aCDI based on capital and replacement costs of $5,000,$10,000, $5,000, and $7,500, respectively. Device lifetimes for CDI,MCDI, i-CDI, and AMX or CMX aCDI of 10, 180, 365, and 180 days are used,respectively.

FIG. 42. pH fluctuation with and without an applied potential in bothCDI (preferential anion adsorption) and i-CDI (no significant differencein ion adsorption at the anode and cathode) cells. Larger pHfluctuations are shown for the CDI cell where two similar surface chargeenhanced electrodes are used.

FIG. 43. Boron removal from solution by converting boron to boratethrough hydroxide creation (higher pH) and charged electrodes. The pH ofthe solution can be modulating through surface charged electrodes orthrough reduction/oxidation of dissolved species.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Shown in FIG. 6 is a cyclic voltammogram (CV) of a pristine carbonxerogel electrode in N₂ deaerated 4.3 mM NaCl solution with a scan rateof 1 mV/s. CVs differ from linear sweep voltammetry in that after theset potential is reached in a CV experiment, the working electrode'spotential is ramped in the opposite direction to return to the initialpotential. The current at the working electrode is plotted versus theapplied voltage (i.e., the working electrode's potential) to give thecyclic voltammogram trace. The CV trace is typically a hysteresis, evenfor perfectly reversible mechanisms. Even reversible couples containpolarization overpotential and thus display a hysteresis trace whenpotential is ramped up from negative to positive and then ramped downfrom positive to negative, passing through an initial potential. Thisoverpotential emerges from a combination of analyte (e.g., ion)diffusion rates and the intrinsic activation barrier of transferringelectrons from the electrode to an analyte. Contrary to most capacitancecurves seen for supercapacitors, which are mostly box-like in theirappearance, CVs in more dilute electrolytes have considerably more“features”. The inventors were the first to realize these features,particularly the relationship of potential of zero charge anddesorption, could be exploited to improve capacitive deionization. Apeak and a trough noted by “PZC” (denoted as “E_(PZC)” herein) are shownin FIG. 6. The peak and the trough denote the locations of the potentialof zero charge, E_(PZC), where an electrode has a minimum in chargestorage or capacitance. If a potential is applied to the electrode toreach this E_(PZC) region, the electrode will have a minimum in ionadsorption. In FIG. 6, to the right of short circuit potential, anionadsorption is promoted on the positive electrode, while to the left ofthe PZC on the negative electrode, cation adsorption is promoted.Therefore, in a capacitive deionization cell, where ion removal is thegoal, the location of the E_(PZC) is crucially important to efficientadsorption of ions; it was not previously recognized that the locationof the E_(PZC) is also crucial to desorption of ions.

Also shown in FIG. 6 is the location of the short-circuit potential fora CDI cell. This location is the potential to which a capacitivedeionization cell will return when the cell is discharged at 0 V (anodesare short-circuited to cathodes). In a CDI cell in which the powersource is a conventional DC power supply or a battery, the positiveterminal is connected to the anode, and the negative terminal isconnected to the cathode. When a potential is applied to the CDI cell,the potential at the anode (positive electrode) will become morepositive, while the potential at the cathode (negative electrode) willbecome more negative. For the anode, when a positive potential isapplied to the anode, the potential will increase from the location atthe short-circuit potential to somewhere in the region highlighted by adotted gray box region on the right. Since these potentials are all morepositive than the E_(PZC), only anion adsorption will be promoted. Atthe cathode, the opposite scenario will unfold. When a negativepotential is applied to this electrode, the potential will decrease fromthe location at the short-circuit potential to somewhere in the regionhighlighted by either a dotted gray box or a solid gray box in theFigures. Since the E_(PZC) is located at a more negative potential thanthe short-circuit potential in this cell, co-ions must be expelled fromthe carbon surface before counter-ion adsorption can take place. Thisco-ion expulsion process at the cathode (i.e., anions attracted to thepositive cathode potential when anode and cathode are short-circuited)results in an inefficiency in the separation process when pristine anduntreated carbon electrodes are used. However, with most standardpotentials being in excess of 1.0 V, there is net salt removal in aconventional CDI process. One of the previously unsolved technicalproblems was why CDI separation fails over a short period of time(25-100 hours of operation, assuming 30 minute cycles) as depicted bythe results in FIG. 4. This question can now be answered by looking at acyclic voltammogram of a CDI cell. Shown in FIG. 7 are overlaid CVs ofpristine and used carbon xerogel electrodes. The used carbon xerogelelectrodes come from the positive (anode) and negative (cathode)electrodes from the CDI experiments depicted in FIG. 4. Immediatelyevident is that the CV of the negative electrode (i.e., the cathode, towhich a negative charging potential is applied) looks remarkably similarto that of the pristine electrode, indicating that very little haschanged on the surface of that electrode. Also apparent is that theE_(PZC) of the negative electrode is located at a similar location tothat of the pristine electrode. However, the situation at the positiveelectrode (anode) is dramatically different. The E_(PZC) has relocatedconsiderably in the positive direction, indicating a permanent change inchemical composition of the anode surface. Using similar arguments toFIG. 6, this relocation of the E_(PZC) at the positive electrode nowresults in an added inefficiency of the CDI process. When chargingpotentials are applied at the positive and negative electrodes of a usedCDI cell after a short-circuit, both electrodes will expel co-ions aswell as adsorbing counter-ions. This mixed adsorption/desorption processresults in a decreased net removal of ions from the input stream,thereby decreasing both the salt adsorption capacity and chargeefficiency. The “j” noted in both FIG. 5 and FIG. 7 is the currentdensity in mA/g of carbon electrode. In these figures, E₊ is thepotential applied to the anode, E⁻ is the potential applied to thecathode, and E_(o) is the potential of the CDI cell at short-circuit.The E_(PZC) relocation during CDI operation provides a key insight intoexplaining the loss in performance found during operation at 1.2/0 V.Voltages separated by a forward slash indicate a charging potential tothe left of the slash, and a discharging potential to the right of theslash. 1.2/0 V charging and discharging potentials produced thedistribution shown in FIG. 8 and in FIG. 7, and reflect the E_(PZC)s ina used CDI cell. As mentioned, during cell operation at 1.2/0 V,degradation in the ion (specifically, Na⁺ and Cl⁻) adsorption capacitywas observed due to cumulative inefficiencies in adsorption/desorptionat both electrodes. The inefficiencies are best explained as two drivingforces that conflict, i.e., one contributes to anion desorption, and theother toward cation desorption. Since η*⁻, the driving force for aniondesorption (i.e., η*⁻=E₀−E_(PZC−), E_(o) being the short-circuitpotential of a CDI cell), already existed in the potential distributionassociated with the pristine electrode (co-ion expulsion in FIG. 6), andthis driving force was maintained even after long-term operation at1.2/0 V (FIG. 7), there remained an inefficiency at the cathode;however, η*₊, the formation of the driving force for cation desorption(i.e., η*₊=E_(PZC+)−E_(o)), and the underlying cause of CDI celldegradation, is related to the relocation of the E_(PZC) for the anodeacross the E_(o) potential (i.e., the E_(PZC) for a pristine anode isslightly negative, but the E_(PZC) for a used anode is very positive).This conclusion is clearly illustrated in FIG. 8 based upon ourpotential distribution and cyclic voltammetry studies.

In assessing the reason behind this relocation of the E_(PZC),especially relocation of the anode E_(PZC), various techniques can beused. The CVs used in FIG. 6 and FIG. 7 are quite sensitive methods fordetecting surface changes, but this method cannot conclusively identifythe particular surface species responsible for this shift in the E_(PZC)in the CV tests carried out here. Therefore, alternative surfaceanalytic methods were employed to analyze the relocation of theE_(PZC)s, including Fourier Transform Infrared Spectroscopy (FTIR) and asurface acidity analysis. Shown in FIGS. 9A and 9B are the FTIR andsurface acidity analyses for carbon xerogel electrodes treated withnitric acid to oxidize the carbon surface. Clearly evident is theincrease in surface oxide groups in the FTIR spectra and surface acidityof the carbon samples after the nitric acid treatment. In FIGS. 10A andB, CVs are shown for both carbon xerogel and Spectracarb (SC)electrodes, which demonstrate positive relocation of the E_(PZC) afteroxidation in nitric acid. These results demonstrate that thechemical/physical reason behind the loss of separation performance in aconventional CDI cell is primarily due to oxidation of the carbon anode(positive electrode). Ultimately, to obtain a stable separation in a CDIcell, shifting of the E_(PZC)s must be controlled at both the anode andthe cathode, with oxdiation being a primary concern for the anode.

FIGS. 11A and 11B compare conventional and inverted capacitivedeionization processes. In conventional CDI, as described above, salt(or other type of ion) is adsorbed onto typically carbon electrodesunder the influence of an applied potential and desorbed when thatpotential is removed, short-circuited, reversed, or reduced. In theinverted capacitive deionization (“i-CDI”) device and method disclosedherein the electrical charging pattern is reversed: salt is desorbedunder an applied potential (where the applied potential is used to reachthe E_(PZC)s at the anode and cathode) and adsorbed when the potentialis removed, short-circuited, reversed, or reduced. This operation ismade possible by the use of surface charged (i.e., surface modified)electrodes at both the anode (positive electrode) and the cathode(negative electrode): as a result of chemically modified electrodesurfaces, anions are preferentially adsorbed at the cathode through theuse of positive surface charge enhanced electrodes and cations arepreferentially adsorbed at the anode through the use of negative surfacecharge enhanced electrodes. In one embodiment, such as that shown inFIG. 11B, the anode can be composed of oxidized or silica-treated carbonelectrodes while the cathode can be composed of pristine oramine-treated carbon electrodes. Other electrode surface modificationchemistries can be used to shift or position the E_(PZC)s of anodes andcathodes, but the surface modification chemistries disclosed herein aresome of the most economical and predictable methods.

Due to the inverted operational scheme depicted for i-CDI in FIG. 11B,it is important to define the working voltage window, aka useablevoltage range, for i-CDI cells. The working voltage window for an i-CDIprocess is shown by the difference in the E_(PZC)s at the anode andcathode. This difference in E_(PZC) location can be used for theadsorption of anions and cations from solution when the potentialrequired to attract ions is far from the E_(PZC)s at the anode andcathode of untreated electrodes. The larger the internal working voltagewindow is, the larger driving force is available foradsorption/desorption of ions, which enables smaller commercial devicesizes as a result of higher salt adsorption capacities at eachelectrode. The size of the working voltage window effectively determinesthe maximum salt adsorption capacity (Γ) of the carbon electrodes.

Shown in FIG. 12 is the working voltage window for an i-CDI cellcomposed of silica-treated carbon xerogel anodes and pristine carbonxerogel cathodes. For this embodiment, the working voltage window isapproximately 0.8 V as the E_(PZC) at the anode is ˜0.62 V and theE_(PZC) at the cathode is ˜−0.17 V (0.62−(−0.17)=0.79 V). On the rightin FIG. 12 are distributed electrode potentials (as measured by a4-electrode study that shows when ˜0.8 V is applied to this i-CDI cell,E₊ and E⁻ at the anode and cathode, respectively) that match thelocations of their respective E_(PZC)s, and thereby make maximum use ofthe working voltage available in this cell. When the cell isshort-circuited (E_(o)), ions are adsorbed in an i-CDI cell as thepotential at each electrode is far from their respective E_(PZC)s. Whilethis exemplary cell shows a working voltage window of 0.8 V, the workingvoltage window can be further expanded for invertedadsorption/desorption performance by creating (by surface modification)electrode materials at the anode that have more positive E_(PZC)s and/orelectrode materials at the cathode that have more negative E_(PZC)s.Shifting or positioning of the E_(PZC) in the negative direction toincrease the working voltage window of this process will be presentedbelow. A working voltage window as small as 0.4 V is operable todeionize input streams; however, as discussed below, the greater theworking voltage window, the greater the adsorption capacity of anembodiment of the invention disclosed herein.

To demonstrate i-CDI operation, a cell was constructed withsilica-modified anodes and pristine carbon cathodes. An identical cellwith pristine carbon electrodes at both the anode and cathode wasconstructed for comparison. Shown in FIGS. 13A and 13B are the currentand conductivity responses for i-CDI and CDI cells operated at 0.8/0 Vfor charging/discharging of the cell. Immediately evident in FIG. 13 isthe inverted conductivity (adsorption/desorption) performance of thei-CDI cell when compared to the conventional CDI when both are exposedto the same molarity of a salt-laden input stream. In an i-CDI cell,salt is desorbed (increase in conductivity) under an applied potentialof 0.8 V and adsorbed (decrease in conductivity) when the cell isshort-circuited or discharged. In addition, there is less charge passed(“Q”) in i-CDI at 0.8 V than in the conventional CDI case, leading to anoverall higher charge efficiency (Λ). A lower Q means that a cell ismore efficient, i.e., electron “use” is lower per mole of ions removedfrom the input stream. Shown in FIGS. 14A, 14B, FIGS. 15A, 15B, 15C,16A, 16B, and FIG. 16C are selected performance characteristics versushours of operation for the i-CDI (FIG. 14A and as denoted in FIGS. 15and 16) and CDI cells (FIG. 14B and as denoted in FIGS. 15 and 16)operated at 0.8/0 V for charging/discharging, and a CDI cell operated at1.2/0 V, with the same adsorption and desorption minutes per cycle.

To examine the long-term stability of the i-CDI process, an i-CDI cellwas operated for 600 hours, with electrical potential cycling betweencharging (0.8 V) and discharging (0 V). Shown in FIG. 17 is the cyclingstability of the i-CDI process as compared to CDI cells operated at1.2/0 V and 0.8/0 V for charging/discharging. The i-CDI process shows animproved lifetime of ≥500% under the conditions tested here,demonstrating the stability of this process when dissimilarsurface-charge enhanced electrodes are used, e.g., a purposefullyoxidized anode (positive electrode) that dramatically mitigates priorperformance degradation inherent in conventional CDI.

Expansion of the i-CDI Working Voltage Window

In order to increase the versatility of the i-CDI process, higher saltadsorption capacities (Γ) can be obtained through expansion of theworking voltage window beyond 0.8 V. This expansion can be accomplishedby either increasing in the positive direction the E_(PZC) at the anode(positive electrode) and/or decreasing in the negative direction theE_(PZC) at the cathode (negative electrode). These directional movementsare purposely performed through the carbon treatments disclosed hereinwith extent of treatment correlating to proper positioning of anE_(PZC). Oxidation of a carbon surface will aid in positively shiftingan E_(PZC), reduction of a carbon surface (decreasing the number ofoxide groups) will aid in negatively shifting an E_(PZC). The data shownin FIG. 10A demonstrates that increased oxidation of a carbon surfacethrough nitric acid treatments at higher temperatures will stepwisepositively shift the E_(PZC). Reduction treatments will likewise cause asimilar effect in the negative direction. In addition to these simpleoxidation/reduction treatments, surface coatings will also play acritical role in the surface charge of the electrode and resultingposition of its E_(PZC), which will define the working voltage window inan i-CDI process.

To demonstrate the effect of surface groups on the E_(PZC) location,both nitric acid and silica surface coatings were used in the inventors'experiments. Shown in FIGS. 18A to 18C are three treatment methods (FIG.18A (heat treatment), FIG. 18B (acid treatment), and FIG. 18C (coatingwith a silica film)) to positively shift the location of E_(PZC). Othermethods known in the art of electrode chemistry exist to accomplishsimilar shifts, including any treatments that yield negatively chargedfunctional groups on an electrode surface, such as the ones shown herefor adding surface oxide and silica groups. Shown in FIGS. 19A and 19Bare tetraethyl orthosilicate (TEOS) and acid treatment methods topositively shift the E_(PZC) of two carbon electrodes: mesoporous carbonxerogel (CX) and microporous Spectracarb (SC).

Shown in FIG. 20 is an example of a treatment process used tofunctionalize carbon electrodes with amine surface groups usingethylenediamine. This treatment process results in —NH₃ ⁺ surfacegroups, a positively shifted pH_(PZC), and a negatively shifted E_(PZC)(as shown in FIG. 21C). When these amine-functionalized cathode carbonelectrodes (P-SC) are combined with anode carbon electrodes withpositively shifted E_(PZC)s (by oxidation treatments or surfacefunctional groups, i.e., N-CX or N-SC), the working voltage window ofthe i-CDI process can be expanded beyond the 0.8 V shown in FIG. 12. Theoriginal working voltage window for the i-CDI process was ˜0.8 V whenusing oxidized/silica treated carbon anodes and pristine carboncathodes.

The inventors have confirmed shifting of the anode E_(PZC) in thepositive direction in both conventional CDI experiments shown in FIG. 7,as well as the i-CDI process shown in FIG. 12, with electrodes noted asN-CX and N-SC in FIGS. 21A-21C for negatively surface chargedelectrodes. An analogous method to shift the cathode E_(PZC) in thenegative direction, increasing the working voltage window in an i-CDIprocess, uses amine treatments with ethylenediamine to create aminefunctional groups on the carbon surface, which will be positivelycharged in an aqueous solution yielding a negatively shifted E_(PZC)(denoted as P-CX and P-SC in FIGS. 21A-21C). As shown in FIGS. 22A-22Band 23A-23B, when the pristine cathode is replaced byamine-functionalized carbon (P-CX or P-SC), the working voltage isincreased to ≥1.0 V. The data shown here is for amine treated carboncathodes, but any surface groups that can negatively shift E_(PZC) (bycreating positive surface charges) will result in the enhanced voltagewindow for an i-CDI process.

For carbon-based electrostatic separations, the role of the E_(PZC) ofthe carbon electrode, or any electrode, is fundamental to a successfulseparation (adsorption of ions from an input stream). In the examplesshown so far, cumulative carbon oxidation of CDI cells has been shown toresult in a positively shifted E_(PZC), which results in a diminishedcapacity for salt removal for conventional CDI. When the E_(PZC) for thecarbon anode is purposefully shifted in the positive direction, eitherthrough oxidation or other surface functional groups (such as silicagroups), this electrode can be paired with a cathode possessing anegatively shifted E_(PZC) (created through reduction or other surfacefunctional groups, such as amine groups) to produce an invertedcapacitive deionization cell in which separation performance does notdegrade with cumulative adsorption/desorption cycles compared toconventional CDI cells. Next, the effect of E_(PZC) on more complicatedcapacitive deionization systems will be shown, further demonstrating theimportance of this parameter in electrostatic separation andimprovements enabled by the disclosed invention.

For comparison to CDI, if an i-CDI cell were implemented with aseparation lifetime of 365 days (compared to 10 days for conventionalCDI shown previously), and the initial and replacement costs were $5000for the unit, this would amount to a 2-year cost of $10,000 since theunit would need to be replaced every year, an obvious improvement overconventional CDI, which had an estimated 2-year cost of $365,000 value.

Asymmetric Membrane Capacitive Deionization

Membrane capacitive deionization (MCDI), as shown in FIGS. 24A-24B and25A-25B, is a modification of traditional CDI that somewhat mitigatesthe diminishing salt adsorption capacity and charge efficiency seen withthe use of conventional CDI cells. With respect to application ofvoltage to cell electrodes from an external power supply, operation ofan MCDI cell is the same as operation of a CDI cell. The structuraldifference between CDI and MCDI cells is the addition of ion exchangemembranes that are coaxial or coplanar with the anode(s) and cathode(s)in a cell. A membrane surrounds an anode or cathode and forms asemipermeable barrier between the input stream and an electrode. BothCDI and MCDI electrostatically concentrate (by adsorption) charged saltcontent (and other ions) from a solution onto the electrostaticallyattractive surfaces of porous carbon electrodes. A traditional CDI cellis formed with porous cathodes and anodes, usually carbonaceousmaterial, separated by a volume of input stream, as described above. InMCDI, complementary anion-attracting and cation-attracting membranes areattached to the anode and cathode, respectively; the membranes form abarrier between each electrode and the solution space. The net effect isan increase in electrosorption capacity due to the enhanced selectivityof adsorption offered by each ion-selective membrane. This increase isaccomplished by each membrane's ability to (i) restrict co-ion transportfrom the carbon electrodes to the input stream and (ii) balancingco-ions which are expelled from the carbon surface with additionalcounter-ions from the input stream via their access through theion-selective membrane. Related art in MCDI technology includes (i) flowelectrode MCDI (EPPAT 2857442 assigned to Korea Institute of EnergyResearch), (ii) potential reversal for cell regeneration (USPAT 8685255assigned to Voltea), and (iii) preparation of anionic exchange membranesfor the mitigation of co-ion repulsion (EPPAT 2641654 assigned toVoltea).

MCDI, as well as CDI, cells are customarily assembled using the same,pristine electrode material for both the cathode and anode electrode;electrodes in capacitive deionization cells must be highly conductiveand porous enough to adsorb a significant quantity of ions. In animprovement over known CDI and MCDI art, the inventions disclosed hereinfor MCDI cells show (1) electrodes with targeted surface functionalgroups capable of hydrolyzing when exposed to an aqueous solution tobecome charged surface groups, and (2) leveraging these charged surfacegroups to effectively attract counter-ions. The inventors havediscovered that charged surface groups shift the position of thepotential of zero charge (E_(PZC)) (as shown in FIG. 9), despite a givenmembrane's ion-selective property, and that MCDI performance can stillbe affected by the E_(PZC) because the solution is in direct contactwith the electrode. The “E_(PZC) shifting” process disclosed above canalso be adapted to improve MCDI performance by identifying the locationof the E_(PZC)s and synergistically combining shifted E_(PZC)s with thefunction of the membrane during the charge and discharge cycles of MCDIoperation. These improved MCDI cells are called asymmetric membranecapacitive deionization (“aMCDI”) cells, and the associated “E_(PZC)shifting” processes are called aMCDI methods.

FIGS. 26A and 26B compare the performance over hours of use oftraditional CDI and MCDI cells formed with carbon xerogel (CX)electrodes. CX electrodes possess a mesoporous structure with a nominalsurface area of ˜200 m²/g. The electrodes are labeled as pristine asthey do not undergo any treatment before the experiments shown here.MCDI obviously outperforms CDI, and shows a larger drop in conductivity.FIG. 27A shows that while CDI may initially have more electronic chargepassed in the ohmic region, MCDI outpaces CDI in the capacitive regionand can also lead to reduced charge leakage as evidenced by the lowerfinal current values. Ultimately, when CDI and MCDI cells arecontinuously cycled, there is more charge passed in the CDI cell, albeitat a lower efficiency due to greater leakage current. Over the testperiod (FIG. 27B), MCDI shows better performance preservation comparedto CDI (similar to the results shown in FIG. 4). Post operation E_(PZC)analysis using impedance spectroscopy shows significant E_(PZC)relocation for the CDI anode while the CDI cathode, MCDI anode, and MCDIcathode show fractional relocation, as shown in FIGS. 28A-28B and29A-29B. This implies that the membrane in MCDI is capable ofmaintaining E_(PZC) positions over cumulative cell cycles.

In a further demonstration of the impact of E_(PZC) location ondeionization, pristine and oxidized CX electrodes were paired to formCDI and MCDI cells. The E_(PZC)s for the pristine and oxidizedelectrodes as identified by electrochemical impedance spectroscopy (EIS)were ˜−0.1 V and +0.5 V vs. SCE electrode, respectively, implying thatthe pristine electrode would naturally adsorb anions whereas theoxidized electrode will naturally adsorb cations in the absence of anapplied electronic charge and at short-circuit conditions. FIGS. 30A and30B show that in contrast to the conventional configuration of usingsimilar pristine electrodes at both the anode and cathode locations,when the CDI and MCDI cells were instead assembled with a pristineelectrode at the anode and the oxidized electrode at the cathode,thereby maximizing the potential for counter-ion excesses within theworking voltage windows, there was increased adsorption over theirrespective, pristine-pristine configurations. Conversely, assemblingeither the CDI or MCDI cell with a pristine cathode and an oxidizedanode resulted in diminished or inverted performance. The currentprofiles for the MCDI cell configurations (FIGS. 31A and 31B) showincreases in electronic charge passed in the order of: pristineanode-oxidized cathode, pristine anode-pristine cathode, and oxidizedanode-pristine cathode. The pristine anode-oxidized cathode MCDIconfiguration is hereafter referred to as asymmetric MCDI (aMCDI), whileits CDI counterpart is asymmetric CDI (aCDI). Bar charts summarizingMCDI results are shown in FIGS. 32A-32C, and indicate that when an aMCDIcell is assembled with a pristine electrode at the anode and an oxidizedelectrode at the cathode, there is as much as a 75% increase in saltadsorption capacity over the respective pristine-only MCDIconfiguration. Compared to a pristine-pristine CDI cell (˜2.5 mg/g),this increase can be as much as 200% in salt adsorption capacity.

In FIGS. 30A-30B, FIGS. 31A-31B, and 32A-32C, when cells are assembledwith a pristine anode and an oxidized cathode, the counter-ion excesseswithin the working voltage window were maximized, and this electrodeconfiguration is considered suitable for an operational mode of chargingat 1.2 V and discharging at 0 V. In contrast, when the cell wasassembled with an oxidized anode and a pristine cathode, co-ion excessesare maximized within the working voltage window. This configuration isoperationally unsuitable for capacitive deionization. In FIG. 30A, aninverted profile is observed for the oxidized anode and a pristinecathode CDI configuration such that the conductivity when discharging islarger than during charging. However, having the membrane in-place (FIG.30B, MCDI configuration) can suppress the inverted profile observed withthe CDI configuration of oxidized anode and pristine cathode. Thisexample of aMCDI (pristine anode-oxidized cathode MCDI configuration)shows that shifting E_(PZC)s in CDI and MCDI operation providessubstantial additional benefits, e.g., significantly improveddesalination performance, when appropriately configured.

E_(PZC) Effect in MCDI with Zorflex® Carbon Electrodes

In order to demonstrate that the E_(PZC)-shifting method for MCDIperformance improvement was not electrode specific, the aMCDI method wasextended to Zorflex® activated carbon cloth electrodes (ZX). Pristine ZXis “as-supplied” ZX, without post-manufacture surface modificationtreatment. Oxidized ZX was synthesized via nitric acid treatment.Pristine and oxidized ZX possess a microporous structure as shown inFIG. 33A, and their respective E_(PZC)'s are (−)0.2 and (+)0.2 V vs. SCEreference electrode (FIG. 33B). The BET surface area for both thepristine and oxidized ZX's is ˜950 m²/g. Four combinations of theelectrodes were used to form MCDI cells with electrode pairings thatincluded: pristine anode-pristine cathode; oxidized anode-pristinecathode; pristine anode-oxidized cathode; and oxidized anode-oxidizedcathode. As with results from the CX electrode MCDI cells (FIG. 33B),when the E_(PZC) was configured in the aMCDI mode (pristine anode(negative Epzc) and oxidized cathode (positive Epzc)), improved ionadsorption was observed (FIGS. 34A and 34B). The highest salt adsorptioncapacity of ˜17 mg NaCl/g ZX was likewise observed for the pristineanode-oxidized cathode cell, where both E_(PZC)'s were outside of thepolarization window (FIGS. 35A and 35B). It was also found that theoxidized anode-oxidized cathode cell performed better than the pristineanode-pristine cathode cell despite both MCDI cells being formed fromidentical electrodes. This was attributed to the proximity of theoxidized E_(PZC) to the short circuit potential (E_(o)). Long-termresults (FIG. 35) show performance stability for all configurations overthe testing period, which can be attributed to the ability of themembrane to localize the E_(PZC) position and maintain performance.

E_(PZC) Effect in MCDI with Spectracarb Carbon Electrodes

In an effort to further improve the performance of aMCDI, it was testedwith high porosity, high surface area (1600 m²/g) Spectracarb (SC)electrodes. SC has a microporous structure (FIG. 36), and pristine SC isas-supplied SC. Oxidized SC was formed via nitric acid treatment, andtheir respective E_(PZC)s were (−)0.1 V and (+)0.3 V vs. SCE referenceelectrode (FIGS. 36A and 36B). Two cell combinations were compared:pristine anode-pristine cathode, and pristine anode-oxidized cathode.The pristine anode-oxidized cathode MCDI (i.e., aMCDI) showed a greaterdrop in conductivity than pristine anode-pristine cathode (FIG. 37A).Its nominal electrosorption capacity was ˜20 mg/g (FIG. 37B). The aMCDIcell also passed more electronic charge, but both cells show near unitycharge efficiency and excellent stability over the test period (FIGS.37C and 37D) and mitigation of E_(PZC) relocation. For comparison toCDI, if an MCDI or aMCDI cell were implemented with a separationlifetime of 180 days (compared to 10 days for conventional CDI shownpreviously), and the initial and replacement costs were $10000 for theunit (higher than CDI or i-CDI due to the inclusion of ion exchangemembranes), this would amount to a 2-year cost of $40,556 since the unitwould need to be replaced every 180 days; while more expensive thani-CDI, aMCDI is still an obvious improvement over conventional CDI,which had an estimated 2-year cost of $365,000 value. This estimate doesnot take into account the salt adsorption capacity, which is still acrucial value, or the salt adsorption rate.

Shifting E_(PZC) Position for Single Membrane Operation

One major disadvantage with MCDI and aMCDI is the requirement formembrane pairs when forming separation cells. However, proper electrodeE_(PZC) position (i) to facilitate specific ion excess adsorption in thepore space and also (ii) to mitigate ion repulsion, is also possibleusing a single membrane aMCDI (i.e., one polarity of electrode(s) in acell is covered with a membrane, and the other polarity of electrode(s)in that cell is not covered by a membrane). Such positioning of theE_(PZC) can be sufficient both to boost and to maintain MCDI performancewhile at the same time providing cost savings in device fabrication.Four cell configurations were constructed with pristine SC anodes andcathodes including CDI, MCDI, CMX only CDI (CMX-CDI), and AMX only CDI(AMX-CDI). A pristine SC (E_(PZC)=−0.1 V) electrode provides an excessof anions at the short-circuit potential (E_(o)), which is also alimitation when that electrode is used as a cathode to adsorb cations at1.2 V. For the single membrane CDI cells, the CMX membrane is used atthe negative electrode, and an AMX membrane is used at the positiveelectrode. FIG. 38A shows that CMX-CDI is capable of providing similarconductivity decrease (i.e., increased ion adsorption) compared to afull MCDI cell. However, the AMX-CDI configuration did not yield anybenefits, but suffered further performance loss, probably as a result ofadditional resistance at the membrane-electrode interface. MCDI wasshown earlier to prevent or lessen E_(PZC) relocation resulting fromelectrochemical reactions. The electrooxidation of the carbon anode isbalanced by dissolved oxygen reduction at the cathode. The membranestarves the cathode of oxygen, thereby correspondingly limiting anodicoxidation. An in-situ probe was used to monitor oxygen response duringcell operation, and as expected, when the cathode was covered with theCMX membrane, minimal perturbation is observed in the dissolved oxygen,and, given the similarity in E_(PZC)s, performance is nearly identicalwith MCDI performance (FIGS. 38A and B). In contrast, for the AMX-CDIcell, performance was worse. Furthermore, as shown in FIG. 38C, theAMX-CDI cell showed the greatest influence of pH to measuredconductivity; either high or low pH imply greater contribution ofhydroxyl or hydronium ions to solution conductivity. Removing thecathode limitation by using an oxidized SC cathode greatly improves theperformance of an AMX-CDI cell. This configuration with asymmetricelectrode is denoted AMX-aCDI (FIGS. 39A-39D). Nonetheless, greaterdissolved oxygen and pH perturbation are still observed when compared toa conventional MCDI cell. The long-term performance of the CDI, MCDI,aMCDI, CMX-CDI, AMX-CDI, and AMX-aCDI cells are compared in FIGS.40A-40C and can be summarized as follows: The aMCDI possesses thehighest capacity and efficiency, but similar performance can be achievedby the AMX-aCDI albeit at lower efficiency due to more limitedmitigation of the parasitic electrochemical reaction as observed fromthe dissolved oxygen profiles. The CMX-CDI cell is capable of providingsimilar performance and efficiency to the standard MCDI cell, while theAMX-CDI displays the worst performance of all combinations.

Distinct from any previous work utilizing porous carbon in a capacitivedeionization cell, the disclosed aMCDI processes (1) utilize electrodetreatment to fabricate electrodes with dissimilar E_(PZC)s, (2) describeelectrode E_(PZC) positions and electrode configurations that lead todramatic performance enhancement, (3) describe synergistic electrodeE_(PZC)-membrane configurations that lead to improved deionizationperformance and salt adsorption capacity, and (4) demonstrate theutilization of E_(PZC) and single membrane cell combinations fordeionization. Also, the performance boosting procedure is not specificto a given type of pristine carbon electrode and can be adapted tovarious manufacturers', and structural types (e.g., cylindrical, fabric,planar, etc.), of electrodes as disclosed herein. These CDI and MCDIcells can be used for removing salt and other ionic content from anytype of input stream, such as power plant wastewater, reservoir feedsfor potable water purification and softening, sea water feeds forpotable water purification and softening, laundry wastewater, feeds forlaboratory water purification, and can be extended to other applicationswhere salt- and/or other ion containing water needs to be deionized,purified, and/or softened. The inventions disclosed herein have broadcommercial implications. Furthermore, the improved electrosorptioncapacity and charge efficiency achieved with the E_(PZC) positioningmethods disclosed herein can substantially reduce energy consumptionduring a deionization operation and lower overall device sizes.

For comparison to CDI, if AMX-aCDI or CMX-aCDI cell were implementedwith a separation lifetime of 180 days (compared to 10 days forconventional CDI shown previously), and the initial and replacementcosts were $75,000 for the unit (somewhat higher than CDI or i-CDI dueto the inclusion of ion exchange membranes), this would amount to a2-year cost of $30,417, since the unit would need to be replaced every180 days, a substantial improvement over conventional CDI, which had anestimated 2-year cost of $365,000 value. AMX or CMX-aCDI would beimplemented instead of i-CDI when capital costs are less of an issue anddevice size and salt removal capacity are more important. This estimatedoes not take into account the salt adsorption capacity, which is stilla crucial value, or the salt adsorption rate; i-CDI andAMX-MCDI/CMX-aCDI both outperform CDI and MCDI in those parameters. Aplot is shown in FIG. 41, which depicts each technology cost as afunction of time with initial estimates of $5,000 per unit for CDI andi-CDI, $10,000 per unit for MCDI, and $7,500 per unit for AMX-MCDI orCMX-MCDI. The projected separation lifetimes for CDI, MCDI, i-CDI, andAMX-MCDI or CMX-MCDI are 10 days, 180 days, 365 days, and 180 days,respectively.

Energy Recovery in CDI, i-CDI, aMCDI, or Other Capacitive-basedSeparations

In all of the separation cells disclosed herein, when a cell is chargedusing an applied potential, charge will be stored at the electrodesurface, regardless of the net ion separation from the bulk solution.This means that during the discharging process when the potential isshort-circuited or reduced (desorption of ions in CDI, adsorption ofions in i-CDI), energy can be recovered in the form of electricalcurrent. While there remain large resistive losses in more dilute saltsolutions, energy recovery can still be quite substantial. By optimizingelectrode surface chemistries and conductivities, saltadsorption/desorption processes can be matched with energy recoveryscenarios (e.g., charging a capacitor, driving a dynamo, driving aninverter, driving an DC/DC converter, operating a pump) therebyoptimizing the energy cost of the separation process. The surfacechemistries noted herein can yield more optimal salt separation fromsolution, either at short-circuit conditions (i-CDI) or with an appliedpotential (aMCDI, AMX-aCDI, and CMX-aCDI), and can be combined withenergy recovery operations by linking discharging and charging cellstogether. A DC/DC converter can be used to efficiently transfer thiselectrical energy and yield a more efficient combined watertreatment/salt separation process.

Modulation of pH with Electrodes Possessing Differing Ion AdsorptionCapacities

Utilizing surface charge enhanced electrodes and purposely positioningthe E_(PZC) of an electrode can be used to effectively modulate the pHof an aqueous solution. For example, if two electrodes with positiveenhanced surface charge (more negative E_(PZC)) are used, the pH underpotential will increase with an applied potential and decrease when thecell is shorted. In conventional CDI cells, when pristine carbonelectrodes are used, there will be a positive surface charge, meaningnatural anion (e.g., chloride) adsorption. Therefore, the cathode willlimit adsorption/separation using this CDI cell where only limitedcation adsorption will take place at the cathode while substantial anionadsorption will take place at the anode. In the bulk solution, sincemore anions will be removed than cations, the pH will increase tomaintain solution electroneutrality. The pH fluctuation for a CDI cellis shown in FIG. 42. Likewise, if two electrodes with negative enhancedsurface charge were used, cation adsorption would be favored, and the pHwould decrease under potential and increase when the cell wasshort-circuited. Finally, if similar amounts of anions and cations areremoved from solution such as in i-CDI cells, pH fluctuations will beminimized. Example pH fluctuations for an i-CDI cell composed ofoxidized anode electrodes and pristine cathode electrodes are also shownin FIG. 42. The pH fluctuations shown are much smaller than in a CDIcell.

By modulating the pH of a solution, various separations can beaccomplished that may be nearly impossible in other water treatmentsystems. For example, boron is a classically difficult compound toremove from solution since it is uncharged in neutral solutions and alsonot hydrated. Since it is not hydrated, it is more difficult for reverseosmosis membrane processes to remove it. However, if the pH is increasedin a solution, boron will be ionized and can subsequently be separatedusing membrane or capacitive-based methods such as CDI, i-CDI, aMCDI,etc. In this manner, if the pH of a solution is increased using anapplied potential and electrodes with positive surface charges orthrough oxygen reduction at the cathode, we can remove boron fromsolution as borate ions. Shown in FIG. 43 is a general schematic forboron conversion to borate using hydroxide creation at the cathodethrough oxygen reduction and subsequent separation from solution usingcharged electrodes.

1. An electrostatic device in a structure comprising: at least oneinlet, at least one outlet, at least one anode, at least one cathode, aswitch operating to apply a short circuit or a user selectable DCconstant voltage or constant current to at least one anode and to atleast one cathode, and an ionic solution admitted through the at leastone inlet and discharged through the at least one outlet, which ionicsolution is deionized by contact with the at least one anode and the atleast one cathode; wherein a location of a potential of zero charge(E_(PZC)) of the at least one anode has been shifted by modification ofa surface of the at least one anode to an increased E_(PZC) value. 2.The electrostatic device of claim 1, wherein the modification of the atleast one anode results from oxidation from heating in the presence ofoxygen.
 3. The electrostatic device of claim 1, wherein the modificationof the at least one anode results from exposure to acid.
 4. Theelectrostatic device of claim 1, wherein the modification of the atleast one anode results from exposure to electrochemical oxidation. 5.The electrostatic device of claim 1, wherein the modification of the atleast one anode results from covalent attachment of functional groupsthat are negatively charged when in contact with the ionic solution andwithout voltage applied to the anode.
 6. The electrostatic device ofclaim 1, wherein the modification of the at least one anode results fromcovalent attachment of silica functional groups.
 7. The electrostaticdevice of claim 1, wherein the modification of the at least one anoderesults from covalent attachment of sulfonic acid groups.
 8. Theelectrostatic device of claim 1, wherein the modification of the atleast one anode results from covalent attachment of any surface groupspossessing net negative surface charges in aqueous solutions.
 9. Anelectrostatic device in a structure comprising: at least one inlet, atleast one outlet, at least one anode, at least one cathode, a switchoperating to apply a short circuit or a user selectable DC constantvoltage or constant current to the at least one anode and to the atleast one cathode, and an ionic solution admitted through the at leastone inlet and discharged through the at least one outlet, which ionicsolution is deionized by contact with the at least one anode and the atleast one cathode; wherein a location of a potential of zero charge(E_(PZC)) of the at least one cathode has been shifted by modificationof a surface of the at least one cathode to a decreased E_(PZC) value.10. The electrostatic device of claim 9, wherein the modification of theat least one cathode results from reduction by exposure to a reducingtreatment from heating in nitrogen, argon, or H₂.
 11. The electrostaticdevice of claim 9, wherein the modification of the at least one cathoderesults from exposure to electrochemical reduction.
 12. Theelectrostatic device of claim 9, wherein the modification of the atleast one cathode results from covalent attachment of functional groupsthat are positively charged when in contact with the ionic solution andwithout voltage applied to the at least one cathode.
 13. Theelectrostatic device of claim 9, wherein the modification of the atleast one cathode results from covalent attachment of amine functionalgroups.
 14. The electrostatic device of claim 9, wherein themodification of the at least one cathode results from covalentattachment of alumina surface species.
 15. The electrostatic device ofclaim 9, wherein the modification of the at least one cathode resultsfrom reduction of specific carbon surface sites.
 16. The electrostaticdevice of claim 15, wherein the specific carbon surface sites are basalplanes.
 17. A method of making an electrostatic device, the methodcomprising: forming a structure comprising at least one inlet, at leastone outlet, at least one anode, at least one cathode, a switch operatingto apply a short circuit or a user selectable DC constant voltage orconstant current to the at least one anode and to the at least onecathode, and an ionic solution admitted through the at least one inletand discharged through the at least one outlet, which the ionic solutionis deionized by contact with the at least one anode and the at least onecathode; wherein a location of a potential of zero charge (E_(PZC)) ofthe at least one anode has been shifted by modification of the anodesurface to an increased E_(PZC) value; a E_(PZC) of the at least onecathode has been shifted by modification of a surface of the at leastone cathode to a decreased E_(PZC) value; or a combination thereof. 18.The method of claim 17, wherein the modification of the at least oneanode results from a treatment selected from the group consisting ofoxidation from heating in a presence of oxygen; exposure to acid;exposure to electrochemical oxidation; covalent attachment of functionalgroups that are negatively charged when in contact with the ionicsolution and without voltage applied to the at least one anode; covalentattachment of silica functional groups; covalent attachment of sulfonicacid groups; and covalent attachment of any surface groups possessingnet negative surface charges in aqueous solutions.
 19. The method ofclaim 17, wherein the modification of the at least one cathode resultsfrom a treatment selected from the group consisting of reduction byexposure to reducing treatment from heating in nitrogen, argon, or H₂;exposure to electrochemical reduction; covalent attachment of functionalgroups that are positively charged when in contact with the ionicsolution and without voltage applied to the at least one cathode;covalent attachment of amine functional groups; covalent attachment ofalumina surface species; and reduction of specific carbon surface sites.20. The method of claim 19, wherein the specific carbon surface sitesare carbon basal planes.